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JT)§'L~7 

BUREAU OF MINES 
INFORMATION CIRCULAR/1989 




Development and Testing of a 
Pneumatic Scraper Blade for 
Conveyor Belt Cleaning 



By C. A. Rhoades, S. G. Grannes, and T. L. Hebble 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Mission: As the Nation's principal conservation 
agency, the Department of the Interior has respon- 
sibility for most of our nationally-owned public 
lands and natural and cultural resources. This 
includes fostering wise use of our land and water 
resources, protecting our fish and wildlife, pre- 
serving the environmental and cultural values of 
our national parks and historical places, and pro- 
viding for the enjoyment of life through outdoor 
recreation. The Department assesses our energy 
and mineral resources and works to assure that 
their development is in the best interests of all 
our people. The Department also promotes the 
goals of the Take Pride in America campaign by 
encouraging stewardship and citizen responsibil- 
ity for the public lands and promoting citizen par- 
ticipation in their care. The Department also has 
a major responsibility for American Indian reser- 
vation communities and for people who live in 
Island Territories under U.S. Administration. 



ft^J^^ . BMfy^^^^) 



Information Circular 9234 



Development and Testing of a 
Pneumatic Scraper Blade for 
Conveyor Belt Cleaning 



By C. A. Rhoades, S. G. Grannes, and T. L. Hebble 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 






Library of Congress Cataloging in Publication Data: 



Rhoades, C. A. (Charles A.) 

Development and testing of a pneumatic scraper blade for conveyor belt cleaning. 

(Bureau of Mines information circular; 9234) 

Supt. of Docs, no.: I 28.27:9234. 

1. Conveyor belts-Cleaning. 2. Blades-Testing. I. Grannes, S. G. (Steven G.) 
II. Hebble, T. L. (Terry L.) III. Title. IV. Series: Information circular (United 
States. Bureau of Mines); 9234. 

TN295.U4 [TN335] 622 s [622\66] 89-600122 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Experimental procedure 3 

Pneumatic cleaning blade mechanisms 12 

Conclusions 12 

ILLUSTRATIONS 

1. Uneven edge wear on blade-type belt cleaners 2 

2. Generic pneumatic blade design 3 

3. Air distribution system 4 

4. Manifold pressure versus time for initial pneumatic blade tests 5 

5. Solid and pneumatic blade edge wear over time 5 

6. Carryback versus manifold pressure 6 

7. Carryback versus contact pressure 7 

8. Pneumatic blade edge wear over time for notched blade 8 

9. Manifold pressure versus time for notched blade 9 

10. Edge wear over time for solid blade 10 

11. Pneumatic and solid cleaner blade service life versus carryback and wear rate 10 

12. Wear rate versus time 11 

13. Dust level measurements 11 

14. Blade wear mechanisms 12 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


cfm 


cubic foot per minute 


mil/yr 


mil per year 


ft 


foot 


fim 


micrometer 


g/ft 2 


gram per square foot 


pet 


percent 


h 


hour 


psi 


pound per square inch 


in 


inch 


s 


second 


mg/m 3 


milligram per cubic meter 







DEVELOPMENT AND TESTING OF A PNEUMATIC SCRAPER BLADE 

FOR CONVEYOR BELT CLEANING 



By C. A. Rhoades, 1 S. G. Grannes, 2 and T. L Hebble' 



ABSTRACT 

A major contributor to the problem of short life expectancy for blade-type conveyor belt cleaners is 
uneven wear along the blade edge. Uneven wear results in the formation of channels in the blade edge, 
which allow material to be carried back between the blade and the belt. In an effort to reduce the 
uneven wear problem, the U.S. Bureau of Mines has studied the mechanisms responsible for effective 
belt cleaning. From this study emerged a design for a cleaner blade that would greatly reduce uneven 
edge wear. The design consists of a standard cleaner blade incorporating air passages that allow for the 
expulsion of air along that part of the blade edge in contact with the conveyor belt surface. The results 
of 18-h tests indicated that the expulsion of air on the blade edge prevents scratches from developing 
into deep grooves. These tests showed that effective blade cleaning life can be extended 25 times using 
the pneumatic cleaning blade, compared with solid metal cleaning blades. 



Mining engineer. 
2 General engineer. 
Metallurgist. 
Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN. 



INTRODUCTION 



Conveyors are used throughout the mining and mineral 
processing industry for transporting high volumes of 
material over relatively short distances. Conveyors offer 
several advantages, including continuous material handling, 
good transport energy efficiency, low labor requirements, 
and high material-handling capacities. However, several 
problems are inherent to conveyor transport. These 
include conveyor adjustment (i.e., belt tracking), system 
reliability, and system spillage. As part of its health and 
safety program, the U.S. Bureau of Mines has undertaken 
a basic study of mechanisms responsible for effective 
conveyor belt cleaning. Improved cleaner blade design can 
reduce exposure of mine personnel to dangerous situa- 
tions, such as explosion, respirable dust, and pinch-point- 
type hazards, resulting from spillage accumulation on or 
beneath troughed conveyor belts. 

The problem of system spillage was recently examined 
as part of a 3-year research effort to determine the mech- 
anisms involved in the operation of blade-type belt clean- 
ers. Significant conclusions drawn in this research were 
that all types of conveyor blade material tend to wear 
unevenly and that wear preferentially occurs in regions of 
the blade not in direct contact with the belt. 4 These 



Rhoades, C. A., T. L. Hebble, and S. G. Grannes. Basic 
Parameters of Conveyor Belt Cleaning. BuMines RI 9221, 1989, 19 pp. 



regions of wear are called wear channels. Wear channels 
occur on numerous scraper blade types, including poly- 
urethane, steel with ceramic inserts, tool steel, and mild 
steel (fig. 1). This research demonstrated that the blade 
wearout phenomena could be slowed by maint ainin g 
proper blade-belt pressures, by eliminating belt irregular- 
ities such as metal splices or recessed belt logos, and by 
increasing blade hardness. With these suggested improve- 
ments, blade cleaning life could be approximately doubled, 
but maximum effective blade life was still generally less 
than 1 day under laboratory conditions. The possibility of 
extending blade life even more led to the consideration of 
the benefits of fluid flow through the wear channels be- 
fore carryback becomes a problem. The fluid would exit 
through the minor wear channels until the surrounding 
blade topography matched the wear channel. Thus, the 
blade would be self-healing. Fluid flow would prevent 
particles from corrupting the blade's cleaning edge. 
Several pneumatic blades were designed and tested, with 
various combinations of slots or holes in the cleaning 
blade edge. Slot widths ranged from 0.016 to 0.250 in. 
The number of holes used in the edge ranged from 6 to 
53, with diameters from 0.016 to 0.125 in. This report de- 
scribes the results of tests with the slotted pneumatic 
blades compared with solid metal cleaning blades, unless 
otherwise stated. 



Polyurethane 




Ceramic Insert 




Tool Steel 



Mild Steel 



Smi 



Figure 1. -Uneven edge wear on blade-type belt cleaners. Mild steel blade is 6 in long. 



EXPERIMENTAL PROCEDURE 



A series of experiments were designed to test the effec- 
tiveness of the pneumatic blade concept. The pneumatic 
blade consisted of a standard cleaner blade incorporating 
passages that allowed for the expulsion of water or air 
along that part of the blade edge in contact with the 
conveyor belt surface. These experiments were designed 
to evaluate optimum fluid flow rates and pressures and 
to develop a theoretical model for the pneumatic blade 
performance. For a description of the test conveyor belt 
and the conveyed test mixture, the reader is referred to the 
previous report. 5 The test procedures used caused highly 
accelerated blade and component wear. 

Preliminary Testing 

Preliminary tests were conducted using a mild (AISI- 
SAE Type 1045) steel blade with a pressurization slot cut 
into the blade surface. Water was selected as the fluid 
because of its low cost, good momentum effects, and 
lubricating properties. The results from these tests were 
favorable. The blade remained flat for 8 h of testing with 
no signs of uneven wear. This was a better result than had 
been achieved with any solid blade previously tested. 
However, water was determined to be impractical for this 
purpose because of serious handling problems in cold 
climates. For this reason, pressurized air was selected as 
the fluid for subsequent tests. 

Testing with air as the fluid was performed using blades 
similar to the water blade (fig. 2). Air was delivered to 



^ork cited in footnote 4. 



Conveyor 

return belt ^CT~^ 




"Airflow 
Figure 2.-Generic pneumatic blade design. 



each pneumatic blade separately by plastic tubing con- 
nected to an air distribution system. The air distribution 
system consisted of a mass-flow meter, four rotameters, 
and four pressure-dampening reservoirs attached to the 
laboratory's 80-psi air line. The flow and pressure could 
be controlled on each blade. The system is shown in 
figure 3. 

Initial tests were conducted at high blade-belt pressures 
(16 psi) to assure air manifold sealing. Carryback of mate- 
rial was negligible for the duration of these tests, averag- 
ing 0.2 g/ft . Figure 4 shows the manifold pressure of the 
blade for the duration of the test. The initial large in- 
crease in pressure during the first half hour indicates the 
"wearing in" period, in which the blade conformed more 
closely to the belt, resulting in a tighter seal between the 
air chamber and the belt. The blade wear that did occur 
was relatively even and appeared to be the result of fine 
particles polishing the surface. Development of wear 
channels would have resulted in a pressure decrease. The 
pressure increase and stabilization over time showed the 
tendency of the blades to wear flat (conforming to the 
belt). Figure 5 compares the cleaning edge of the pneu- 
matic blade after 30 h of service with solid metal cleaner 
blades after 18 h on the test conveyor. All blades were 
made from AISI-SAE Type 1045 carbon steel. The solid 
blades were removed from service because the carryback 
had increased to over 5 g/ft 2 . The pneumatic blade carry- 
back was less than 0.6 g/ft 2 when the blade was removed 
from service. Although some smooth contours existed at 
the ends, the general shape was not consistent with chan- 
nel wear. 

After 30 h of run time in the preliminary test, no evi- 
dence of channel formation was apparent, negligible carry- 
back was observed, and manifold pressures were holding 
at a high level. It appeared as if the test would continue 
indefinitely until the slot depth was consumed by the flat 
polishing wear that was occurring. A subsequent test was 
designed to determine whether existing wear channels 
could be flattened by the pneumatic blade. In this test, 
artificial wear channels were machined across the blade 
edge. Five severe channels were machined to a depth of 
0.010 in and a width of 0.125 in. A 1-h series of prelimi- 
nary setup tests were performed to determine airflow 
effectiveness limits and blade-belt contact pressure on 
cleaning. Figure 6 shows the effectiveness of air in reduc- 
ing carryback at constant blade-belt pressures of 35 and 
21 psi. Notice that carryback was significant for both trials 
with no airflow but was reduced as airflow rates were in- 
creased. Figure 7 shows the effect of contact pressure on 
carryback. Carryback was essentially zero (immeasurable) 
for contact pressures of 20 to 25 psi. 



Long-Term Wear Testing 

Based on these tests, a long-term wear test to deter- 
mine the pneumatic blade's ability to correct the artificial 
wear channels was designed. The blade-belt pressure for 
this test was set at 20 psi, and the manifold pressure was 
set at 20 psi. Figure 8 shows the profile of blade 2 during 
the 21-h test. Figure 9 shows the manifold pressures over 
the test interval. Notice the gradual smoothing of the 
blade and the corresponding increase in manifold pres- 
sures. Figure 10 shows a typical solid cleaner blade with 
time. The troughs form and widen as time goes on. The 
solid blades were removed from service between 12 and 
18 h because the carryback increased to over 5 g/ft 2 . The 
pneumatic blade did not develop the wear pattern of the 
solid blade. The air being forced through the blade pre- 
vented abrading and eroding particles from localized 
attack on the blade. This is shown in figure 8 by the even 
wear of the blade near the air slot. The comparison of 
the two blades in figures 8 and 10 shows the "self-healing" 
nature of the pneumatic blade by the gradual total surface 
wear and the elimination of the starting notches. The 
pneumatic blade experiment was stopped at 21 h with no 
indications of uneven wear. The carryback was less than 
0.6 g/ft 2 . 



Because of flat wear, the service life of the pneumatic 
blade is significantly greater than that of conventional solid 
blades. Figure 11 contrasts the carryback amounts and 
wear rates for conventional solid blades and the pneumatic 
blade. Wear channels in the solid blade cause a significant 
carryback amount of 2 g/ft 2 after 5 h and 6 g/ft 2 in 24 h. 
These wear channels are illustrated in figure 10. The 
pneumatic blade carryback remained stable at 0.5 g/ft 2 - 
one-tenth the carryback of the solid blade at the conclu- 
sion of the test interval. Although the pneumatic blade 
has a 33-pct higher average wear rate, the wear pattern is 
flat (figs. 5, 8). This evidence suggests that the service life 
of the pneumatic blade is limited only by the air chamber 
depth and would be about 500 h under the accelerated test 
conditions. Since wear rates can be decreased by using 
harder steels, this service life could be further extended. 

The pneumatic blade wear rate is affected by the total 
airflow through the system and through each individual 
blade. The wear data in figure 12 show the decreasing 
wear rate for increased airflow. The airflow was not in- 
creased beyond 34 cfm because of the dust created and to 
prevent the lifting of the blade's cleaning edge away from 
the belt. A balance of airflow, surface treatment, and de- 
sign should optimize the cleaner blade for each individual 
conveyor and the material conveyed. 




Figure 3.-Air distribution system. 




Figure 4.-Manifold pressure versus time for initial pneumatic blade tests. 



b m 



Figure S.-Solid and pneumatic blade edge wear over time. A, Solid blade, 18 h; B, pneumatic blade, 30 h. 



KEY 
Contact pressure 

21 psi 

35 psi 








5 10 15 

MANIFOLD PRESSURE, psi 



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Figure 8. -Pneumatic blade edge wear over time for notched blade. A, h; B, 6 h; C, 1 1 h; D, 21 h. 



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Figure 11. -Pneumatic and solid cleaner blade service life 
versus carryback and wear rate. 



6 dust 
The 



Dust Generation 

One additional parameter noted during the testing 
was dust generation. The research conveyor was housed 
in a 50- by 75- by 15-ft building. A GCA RAM-l 
monitor was used to measure respirable dust levels, 
operation and performance of this instrument have been 
described in the literature. 7 The instrument can be oper- 
ated in three concentration ranges of to 2.0, to 20, and 



Reference to specific products does not imply endorsement by the 
U.S. Bureau of Mines. 

Lilienfeld, P. Improved Light Scattering Dust Monitor (contract 
HO377092, GCA Corp.). BuMines OFR 90-79, 1979, 48 pp.; NTIS PB 
299-938. 

Williams, K. L., and R. J. Timko. Performance Evaluation of a 
Real-Time Aerosol Monitor. BuMines IC 8968, 1984, 20 pp. 



to 200 mg/m 3 , and in four measurement time constants 
of 0.5, 2, 8, and 32 s. The RAM-l was operated with a 
cyclone precollector for particle size selection, which 
permitted particles less than 10 /im in diameter. Calibra- 
tion was done with an Arizona road dust. The monitor 
was calibrated before and after each run. A time constant 
of 8 s and a range of to 20 mg/m 3 were used. The mon- 
itor was placed 10 ft in front of the head pulley and 10 ft 
above the floor. Figure 13 shows the respirable dust levels 
during a 3-h test run with a solid blade and three pneu- 
matic blades. The pneumatic blades had either six 0.125- 
in-diameter holes, fifty-two 0.016-in-diameter holes, or a 
0.026-in continuous slot. The dust levels increased to a 
maximum of 2.5 mg/m 3 during the 3-h run. These levels 
are well below the maximum level of 5 mg/m 3 allowed by 
the U.S. Mine Safety and Health Administration and the 
U.S. Occupational Safety and Health Administration. 



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Figure 13.-Dust level measurements. 



12 



PNEUMATIC CLEANING BLADE MECHANISMS 



Two mechanisms were found to contribute to uneven 
blade wear: particle wedging and viscous material flow. 
These mechanisms contribute to a blade-belt contact 
interface that could be described as being in a state of 
unstable equilibrium. The blade-belt interface will remain 
flat as long as no external disturbance occurs, such as a 
random particle gouge or a belt imperfection that causes 
a wear channel to begin. The concept of the pneumatic 
blade originated as a method of bringing the contact 
interface into a state of stable equilibrium. The idea was 
that a fluid emanating from the blade-belt interface would 
stabilize the contact interface by preventing preferential 
particle wedging and viscous material flow. The momen- 
tum and pressure of the fluid should preferentially restrict 
abrasive particles from entering possible wear channel 
growth areas. The fluid can be thought of as stabilizing 
the scraper blade cleaning process but not doing the actual 
cleaning. These concepts are illustrated in figures 2 and 
14. 

If the pressure of the air within the manifold is kept 
within certain limits relative to the blade-to-belt contact 
pressure, air will be expelled along scratches, preventing 
further entrapment of abrasive particles. The parameters 
critical to the efficient operation of the pneumatic cleaner 
blade include (1) the blade-to-belt contact pressure, (2) the 
air pressure used in the blade, and (3) the shape and size 
of the air exit hole in the blade edge. 

The blade-to-belt contact pressure must be kept sig- 
nificantly above the critical pressure 8 to avoid intermittent 
loss of cleaning efficiency because of fluctuations in air 
pressure and irregularities in the drum and belt surface. 
If the contact pressure is near the critical pressure, fluctua- 
tions in pressure and belt and drum surface could be 
sufficient to allow enough separation of the blade from 
the belt to result in the loss of significant quantities of air. 



Conveyor 

return belt </^\ 




Uneven blade wear 
erosion trough 



Figure 14.-Blade wear mechanisms. 

The problem with using solid blades at pressures sig- 
nificantly above the critical contact pressure has been the 
high friction between the blade and belt, resulting in high 
drive motor current draw. With pneumatic blades, how- 
ever, high contact pressure is not as much of a problem 
because the escaping air acts as a lubricant, which reduces 
the friction. 

The pressure of the air expelled along the contact edge 
of the blade must be kept below the blade-to-belt contact 
pressure. Air pressures at, or above, the contact pressure 
would result in the blade's being lifted off the belt. This 
would result in two problems: The edge of the blade 
would not be in position to scrape the carryback off the 
belt, and the gap between the blade and belt would allow 
the escape of large quantities of air as well as generation 
of excessive dust. 



CONCLUSIONS 



The results of the testing program have shown that 
the pneumatic blade design can prevent the creation and 



T"he minimum pressure for which incremental pressure increases do 
not result in increased cleaning effectiveness. 



enlargement of wear channels on the cleaner blade edge, 
which occur on conventional solid blades. While the 
overall wear rate is slightly higher, the wear occurs evenly 
along the edge. This even wear results in a considerably 
longer useful life expectancy for the pneumatic blade 
design. 



INT.BU.OF MINES,PGH.,PA 29033 



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